P0138

DTC P0138

CIRCUIT DESCRIPTION
The DTC P0138 Heated Oxygen Sensor (HO2S) Circuit High Voltage Sensor 2 diagnostic monitors the voltage on the signal circuit of the post catalyst oxygen sensor. In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the heated oxygen sensors HO2S 1 and HO2S 2. The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2.

DTC DESCRIPTOR
This diagnostic procedure supports the following DTC:
DTC P0138 Heated Oxygen Sensor (HO2S) Circuit High Voltage Sensor 2

CONDITIONS FOR RUNNING THE DTC
- DTC P0031, P0032, P0100, P0101, P0102, P0103, P0110, P0112, P0113, P0115, P0117, P0118, P0120, P0121, P0122, P0123, P0125, P0171, P0172, P0300, P0301-P0304, P0335, P0340, P0341, P0385, P0441, P0442, P0446, P0456, P0500 are not set.
- The engine coolant temperature (ECT) is more than or equal to 70°C (158°F).
- The engine speed is more than 1,600 RPM.
- The target air fuel ratio is LEAN.
- DTC P0138 runs continuously when the above conditions are met.

CONDITIONS FOR SETTING THE DTC
The average HO2S 2 voltage is more than 590 mV for more than 30 seconds.

ACTION TAKEN WHEN THE DTC SETS
- The PCM illuminates the malfunction indicator lamp (MIL).
- The PCM stores the conditions which were present when the DTC set as Freeze Frame data.
- The PCM will turn the HO2S 2 heater OFF.

CONDITIONS FOR CLEARING THE MIL/DTC
- The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
- A DTC clears after 40 consecutive warm-up cycles have occurred without a fault.
- A DTC can be cleared by using the scan tool Clear DTC Information function.

DIAGNOSTIC AIDS
Check for the following conditions:
- The HO2S 2 input voltage on a cold sensor, ignition ON with the engine OFF, is approximately 55 mV.
- Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 2 display on the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the HO2S 2 display will indicate the location of the fault.
- If a DTC P0138 is intermittent, driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. Refer to Inspection/Maintenance (I/M) Heated Oxygen Sensor/Oxygen Sensor (HO2S/O2S) System Set Procedure. Heated Oxygen Sensor/Oxygen Sensor (HO2S/O2S) System Set Procedure
- An intermittent malfunction may be caused by fault in the HO2S circuit sensor electrical circuit. Inspect the wiring harness and components for an intermittent condition.
- Repair any electrical circuit faults that were found.

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

TEST DESCRIPTION

Step 1 - Step 9:

The numbers below refer to the step numbers in the diagnostic table.
1. The Diagnostic System Check - Vehicle prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool, if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. This step checks whether the malfunction is a high input condition.
3. This step determines if the DTC P0138 is the result of a hard failure or an intermittent condition.